editing save files in Heroes of Might and Magic III: The Restoration of Erathia, complete collection.

I will keep repeating: the first rule for a competent player is to regularly create save files, starting from the very beginning. In Campaigns, do this for each scenario. Let's give the save files "descriptive" names so that you can easily find what you need later, even after a year or five. The save file name should include: the campaign name (abbreviated), the scenario number, the month, week, and day. For example, "non3-231.CGM": "New Beginning," 3rd scenario, 2nd month, 3rd week, 1st day. Periodically move save files from the GAMES folder to an archive and clean the folder, leaving only the most recent ones (about a week's worth). Then, in the worst-case scenario, you will only have to replay a few days.
You need to be especially careful with secondary skills: it's a good idea to find out before you start which skills are necessary or most useful for the main hero throughout the campaign and in individual scenarios. It's also worth finding out before the game starts which bonus to choose from the ones offered at the beginning: this can greatly affect the course of the game and its success or failure.
In your case, try replaying the last scenario (where the main hero still has a free skill slot, although the game may not give you what you need :().

Help, I can't sleep!

Try reading a textbook on calculus before going to bed! ;) :D